Java Reference
In-Depth Information
person (firstname, lastname)>
,
<!ELEMENT lawyer (firstname,
lastname)>
, and
<!ELEMENT accountant (firstname, lastname)>
.
Eachelementcontainsrepeatedchildelementcontent.Ifyouneedtoaddanotherchild
element(suchas
middleinitial
),you'llneedtomakesurethatalltheelementsare
updated; otherwise, you risk a malformed DTD. Parameter entities can help you solve
this problem.
Parameterentitiesareclassifiedasinternalorexternal.An
internal parameter entity
isaparameterentitywhosevalueisstoredintheDTD,andhastheform
<!ENTITY %
name value
>
,where
name
identifies theentity and
value
specifies itsvalue. For
example,
<!ENTITY % person-name "firstname, lastname">
declares
a parameter entity named
person-name
with value
firstname, lastname
.
Oncedeclared,thisentitycanbereferencedinthethreepreviouselementdeclarations,
as follows:
<!ELEMENT salesperson (%person-name;)>
,
<!ELEMENT
lawyer (%person-name;)>
, and
<!ELEMENT accountant (%person-
name;)>
.Insteadofadding
middleinitial
toeachof
salesperson
,
lawyer
,
and
accountant
, as was done previously, you would now add this child element
to
person-name
, as in
<!ENTITY % person-name "firstname,
middleinitial, lastname">
, and this change would be applied to these ele-
ment declarations.
An
external parameter entity
isaparameterentitywhosevalueisstoredoutsidethe
DTD.Ithastheform
<!ENTITY %
name
SYSTEM
uri
>
,where
name
identifies
the entity and
uri
locates the external file. For example,
<!ENTITY % person-
name SYSTEM "http://www.tutortutor.ca/entities/names.dtd">
identifies
names.dtd
asstoringthe
firstname, lastname
texttobeinsertedin-
toaDTDwherever
%person-name;
appearsintheDTD.Thealternative
<!ENTITY
%
name
PUBLIC
fpi uri
>
form can be specified.
Note
ThisdiscussionsumsupthebasicsofDTD.Oneadditionaltopicthatwasnot
covered(forbrevity)is
conditional inclusion
,whichletsyouspecifythoseportionsof
aDTDtomakeavailabletoparsers,andistypicallyusedwithparameterentityrefer-
ences.
XML Schema
XML Schema
isagrammarlanguagefordeclaringthestructure,content,and
semantics
(meaning) of an XML document. This language's grammar documents are known as
schemas that are themselves XML documents. Schemas must conform to the XML
